High porosity acoustic backing with high thermal conductivity for ultrasound tranducer array

ABSTRACT

A backing block for an ultrasonic transducer array stack of an ultrasound probe is formed as a composite structure of graphite foam impregnated with an epoxy resin. The epoxy resin penetrates the porous foam structure at least part-way into the depth of the graphite foam block and, when cured, provides the backing block with good structural stability. The composite graphite foam backing block is bonded to the integrated circuit of a transducer to provide high thermal conductivity away from the transducer and good acoustic attenuation or scattering of rearward acoustic reverberations.

This invention relates to medical diagnostic ultrasound systems and, inparticular, to backing materials for an ultrasonic transducer array.

Two dimensional array transducers are used in ultrasonic imaging to scanin three dimensions. Two dimensional arrays have numerous rows andcolumns of transducer elements in both the azimuth and elevationdirections, which would require a large number of cable conductors tocouple signals between the probe and the mainframe ultrasound system. Apreferred technique for minimizing the number of signal conductors inthe probe cable is to perform at least some of the beamforming in theprobe in a microbeamformer ASIC (application specific integratedcircuit.) This technique requires only a relatively few number ofpartially beamformed signals to be coupled to the mainframe ultrasoundsystem, thereby reducing the required number of signal conductors in thecable. However a large number of signal connections must be made betweenthe two dimensional array and the microbeamformer ASIC. An efficient wayto make these connections is to design the transducer array and the ASICto have flip-chip interconnections, whereby conductive pads of thetransducer array are bump bonded directly to corresponding conductivepads of the ASIC.

The high density electronic circuitry of the microbeamformer ASIC can,however, produce a significant amount of heat in its small IC package,which must be dissipated. There are two main directions in which thisheat can flow. One direction is forward through the acoustic stacktoward the lens at the patient-contacting end of the probe. Thermalconductivity is aided in this direction by electrically conductiveelements in the transducer stack. This forward path exhibits relativelylow resistance to thermal flow. Build-up of heat in the lens must thenbe prevented by reducing transmission voltage and/or the pulserepetition frequency, which adversely affects probe performance.

The preferred thermal conduction direction is to the rear, away from thelens and toward a heat spreader (typically aluminum) at the rear of theprobe. But generally located behind the transducer stack, the arrayelements and the microbeamformer ASIC, is an acoustic backing block. Thepurpose of the acoustic backing block is to attenuate ultrasonic energyemanating from the rear of the acoustic stack and prevent this energyfrom causing reverberations that are reflected toward the acousticstack. An acoustic backing block is generally made of a material withgood acoustic attenuation properties such as an epoxy loaded withmicro-balloons or other sound-deadening particles. Although manyepoxy-filler composite backings have been developed to isolate the ASICsfrom the supporting structure (usually aluminum) of the probe assembly,they have two disadvantages. If formulated to have high attenuation thenthey have unacceptable thermal conductance. If formulated to have highthermal conductance they have unacceptable attenuation. Hence it isdesirable to provide an acoustic backing block for an ultrasound probewhich exhibits good acoustic attenuation of acoustic energy entering theblock, good thermal conductivity toward the rear of the probe and awayfrom the lens, good structural stability which can support the acousticstack as needed, and appropriate electrical isolation of themicrobeamformer ASIC from other conductive components of the probe.

In accordance with the principles of the present invention, a backingblock for an ultrasonic transducer array stack is formed of a porousgraphite foam material which has high acoustic attenuation and highthermal conductivity. In a preferred implementation the foam backingblock is constructed as a composite with the foam structure filled withan epoxy resin. An electrically isolating layer can be located on thetop of the backing block at the bond between the backing block and theASIC of the acoustic stack assembly.

In the drawings:

FIG. 1 illustrates an acoustic stack with a thermally conductive backingblock constructed in accordance with the principles of the presentinvention.

FIG. 2 illustrates the acoustic stack of FIG. 1 when assembled in atransducer probe with a lens cover.

FIG. 3 is a perspective view of a thermally conductive backing blockconstructed in accordance with the principles of the present invention.

FIG. 4 is a top plan view of a thermally conductive backing blockconstructed in accordance with the principles of the present invention.

FIG. 5 is a side cross-sectional view of a thermally conductive backingblock constructed in accordance with the principles of the presentinvention.

FIG. 6 illustrates a composite foam backing block constructed inaccordance with the principles of the present invention.

FIG. 7 illustrates an acoustic stack assembly of the present inventionwith a film insulating layer between the ASIC and a composite foambacking block.

FIG. 8 illustrates an acoustic stack assembly of the present inventionwith a parylene-coated composite foam backing block.

Referring first to FIG. 1, an acoustic stack 100 with a thermallyconductive backing block which is constructed in accordance with theprinciples of the present invention is shown schematically. Apiezoelectric layer 110 such as PZT and two matching layers bonded tothe piezoelectric layer are diced by dicing cuts 75 to form an array 170of individual transducer elements 175, four of which are seen in FIG. 1.The array 170 may comprise a single row of transducer elements (a 1-Darray) or be diced in two orthogonal directions to form atwo-dimensional (2D) matrix array of transducer elements. The matchinglayers match the acoustic impedance of the piezoelectric material tothat of the body being diagnosed, generally in steps of progressivematching layers. In this example the first matching layer 120 is formedas an electrically conductive graphite composite and the second matchinglayer 130 is formed of a polymer loaded with electrically conductiveparticles. A ground plane 180 is bonded to the top of the secondmatching layer, and is formed as a conductive layer on a film 150 of lowdensity polyethylene (LDPE) 140. The ground plane is electricallycoupled to the transducer elements through the electrically conductivematching layers and is connected to a ground conductor of flex circuit185. The LDPE film 150 forms the third and final matching layer 140 ofthe stack.

Below the transducer elements is an integrated circuit 160, an ASIC,which provides transmit signals for the transducer elements 175 andreceives and processes signals from the elements. Conductive pads on theupper surface of the integrated circuit 160 are electrically coupled toconductive pads on the bottoms of the transducer elements by stud bumps190, which may be formed of solder or conductive epoxy. Signals areprovided to and from the integrated circuit 160 by connections to theflex circuit 185. Below the integrated circuit 160 is a backing block165 which attenuates acoustic energy emanating from the bottom of thetransducer stack. In accordance with the principles of the presentinvention, the backing block also conducts heat generated by theintegrated circuit away from the integrated circuit and the transducerstack and away from the patient-contacting end of the transducer probe.

FIG. 2 illustrates the transducer stack assembly of FIG. 1 whenassembled inside a transducer probe. In the probe of FIG. 2 the thirdmatching layer 140 is bonded to the acoustic lens 230. Ultrasound wavesare transmitted through the lens 230 and into the patient's body duringimaging, and echoes received in response to these waves are received bythe transducer stack through the lens 230. The LDPE film 150 serves toenclose the transducer stack in this embodiment as it is wrapped aroundthe stack and bonded by an epoxy bond 210 to the probe housing 220.Further details of this construction are found in US patent publicationno. US 2010/0168581 (Knowles et al.)

A preferred implementation for the backing block 165 is illustrated inthe remaining drawings. A preferred backing block 165 starts with ablock of graphite 20. Other alternatives include graphite loaded withmetals such as nickel or copper which provide good machinability andfavorable thermal properties. The graphite block 20 is used to form acomposite backing structure which satisfies a number of performanceobjectives. First, the backing structure must have good Z-axis thermalconductivity. Graphite has good thermal conductivity, a Tc of 80 to 240W/m° K at 0° C.-100° C. For conduction parallel to the crystal layers,Tc will approach 1950 W/m° K at 300° K. The Z-axis direction is thedirection back and away from the transducer stack 100 and the integratedcircuit 160. Thus, it is desirable to align the crystal layers of thegraphite block 20 for heat flow in the Z-axis direction. In otherimplementations it may be desirable to preferentially conduct heatlaterally or both laterally and in the Z-axis direction, in which case adifferent direction of crystal alignment may be desired or the alignmentdirection may be immaterial to the design. When aluminum is used todissipate some of the heat, which may be by use of an aluminum heatspreader or an aluminum frame inside the probe housing, it is desirablefor the thermal conductivity of the backing block be comparable to orbetter than that of aluminum, so that heat will preferentially flow tothe aluminum. Aluminum has a comparable Tc of 237 W/m° K at roomtemperature, so this performance objective is well met by a graphiteblock 20.

A second objective is that the backing block provide structural supportfor the acoustic stack 100 and integrated circuit 160. A graphite blockis structurally sound, satisfying this objective.

A third objective is to provide electrical isolation of the integratedcircuit 160 from the aluminum member or frame of the probe. Graphite,being electrically conductive, can satisfy this objective by coating thebacking block with a non-conductive insulative coating. In someimplementations it may be desirable to coat only the side of the blockwhich is in contact with the transducer stack. In other implementationsit may be desirable to coat multiple sides of the backing block. It maybe desirable, for instance, to coat the lateral sides of the block withan insulative acoustic damping material which would provide theadditional benefit of suppressing lateral acoustic reverberation.

The fourth objective is that the backing block must dampen acousticenergy entering the block. Graphite is a good conductor of acousticenergy and provides very little inherent acoustic damping. Thisobjective is satisfied by employing the graphite block as the frameworkfor a composite structure of internal acoustic dampening members asshown in FIGS. 3, 4, and 5. In these drawings the graphite is renderedtranslucent for clarity of illustration of the internal compositestructure of the block. The dampening members are formed as a pluralityof angled cylinders 30 of backing material in the backing block. Thecylinders 30 are cut or drilled into the graphite block 20, then filledwith acoustic dampening material such as epoxy filled with microballoons or other acoustic damping particles. As the top plan view ofthe backing block of FIG. 4 illustrates, the tops of the cylinders 30present a large area of acoustic dampening material to the back of theintegrated circuit. A considerable amount of the undesired acousticenergy emanating from the back of the integrated circuit and acousticstack will thus pass immediately into the dampening material. Theangling of the cylinders as seen in FIG. 3 and best seen in thecross-section view of FIG. 5 assures that acoustic energy traveling inthe Z-axis direction will have to intersect dampening material at somepoint in the path of travel. Preferably, there is no path in the Z-axisdirection formed entirely of graphite, and the angling of the cylindersdoes not promote reflection of energy back to the integrated circuit butprovides scattering angles downward and away from the integratedcircuit. In practice it may be sufficient to block most of the Z-axispathways such as by blocking 95% of the pathways. Thus, the angling ofthe cylinders assures damping of all or substantially all of the Z-axisdirected energy.

Heat, however, will find continuous pathways through the graphitebetween the cylinders 30. Since the flow of heat is from highertemperature regions to lower (greater thermal density to lesser), heatwill flow away from the integrated circuit 160 and acoustic stack 100 tostructures below the backing block 165 where it may be safelydissipated.

Other materials may be used for the thermally conductive material of thebacking block, such as aluminum, graphite foam, or aluminum nitride. Onecomposite structure which has been found to be advantageous for manyapplications is a conductive graphite foam filled with epoxy resin. Themacroscopic nature of the machined and filled graphite block describedabove can provide an uneven bonding surface to the ASIC, which isvulnerable to expansion mismatches. the machining and filling of theholes with epoxy is also a labor intensive process. FIG. 6 illustratesan implementation of the present invention in which The backing materialof the backing block of FIG. 6 uses a thermally conductive graphite foam(POCO HTC) filled with a soft unfilled attenuating epoxy resin. Theunfilled HTC foam has significant porosity (60%), of which 95% of thetotal porosity is open. When this open porosity is filled with softresin, this composite backing exhibits a high acoustic attenuation ofapproximately 50 dB/mm at 5 Mhz. This high attenuation is mainly due totwo mechanisms: 1) the absorption of acoustic energy by the soft resinand 2) acoustic energy scattering due to the impedance mismatch betweenepoxy, graphite, and air in the porous structure. As a result of thishigh acoustic attenuation, the backing thickness can be reduced tofacilitate transducer heat dissipation. Another property of this epoxyfilled graphite foam is its high thermal conductivity (˜50 W/mK), whichis one order of magnitude higher than typical epoxy-filler backingformulations.

The composite graphite foam backing block 32 of FIG. 6 illustrates thehigh porosity of the foam. In this example the surface of the foam block32 is coated with an epoxy resin 34 which soaks into the block by adepth 36 which is a function of the porosity of the foam block and theviscosity of the resin, as indicated by the shaded areas in the drawing.The cured epoxy gives the block good structural stability. The compositebacking block can then be directly bonded to the ASIC 160 with a thinepoxy bondline. In order to provide adequate electrical isolation fromthe ASIC, an insulating layer can be used between the backing block andthe ASIC as illustrated in FIGS. 7 and 8, which show exploded views oftwo implementations in an acoustic stack. At the top of each drawing isthe transducer layer 170 with its matching layers. Below the transducerlayer is the ASIC 160. In FIG. 7 a thin (12 to 25 microns) polyimidefilm 38 is attached to the ASIC before bonding the backing block to theassembly. The composite foam backing block 32 is then bonded to theinsulating film 38. In FIG. 8 a parylene coating 58 of 10 to 15 micronsis applied to the HTC backing block. The parylene coated backing blockis then bonded to the ASIC 160.

1. An ultrasonic transducer array assembly comprising: an array oftransducer elements having a forward desired direction for thetransmission of ultrasonic waves and a rearward undesired ultrasonicemission direction; an integrated circuit structurally coupled to thearray of transducer elements; a composite foam backing block, locatedrearward of the array of transducer elements and integrated circuit, thecomposite foam backing block being formed of a foam material having ahigh thermal conductivity and a porous structure; and an epoxy resinfilling at least some of the porous structure of the foam backing block,wherein ultrasonic emissions in the rearward direction is scattered orattenuated by the porous foam structure and epoxy, and heat is conductedaway from the array of transducer elements and integrated circuit by thebacking block material.
 2. The ultrasonic transducer array assembly ofclaim 1, wherein the foam material further comprises a graphite foam. 3.The ultrasonic transducer array assembly of claim 1, wherein thecomposite foam backing block further comprises an exterior surface, andwherein the epoxy resin fills the porous structure of the foam backingblock adjacent to the exterior surface.
 4. The ultrasonic transducerarray assembly of claim 1, wherein the integrated circuit furthercomprises a beamformer ASIC coupled to the rearward side of the array oftransducer elements, and wherein the composite foam backing block isthermally coupled to the beamformer ASIC.
 5. The ultrasonic transducerarray assembly of claim 4, wherein the composite foam backing block isbonded to the beamformer ASIC by an epoxy bond.
 6. The ultrasonictransducer array assembly of claim 4, further comprising an electricallyinsulating layer between the beamformer ASIC and the composite foambacking block.
 7. The ultrasonic transducer array assembly of claim 6,wherein the electrically insulating layer further comprises a polyimidefilm.
 8. The ultrasonic transducer array assembly of claim 7, whereinthe polyimide film is no thicker than 25 microns.
 9. The ultrasonictransducer array assembly of claim 6, wherein the electricallyinsulating layer further comprises a parylene coating.
 10. Theultrasonic transducer array assembly of claim 9, wherein the parylenecoating is no thicker than 15 microns.
 11. The ultrasonic transducerarray assembly of claim 1, wherein the porous structure exhibits aporosity of at least 60%.
 12. The ultrasonic transducer array assemblyof claim 11, wherein at least 95% of the total porosity of the porousstructure is open.
 13. The ultrasonic transducer array assembly of claim1, wherein the rearward ultrasonic emission scattering is due to theimpedance mismatch between epoxy, the porous foam material, and air inthe porous foam structure.
 14. The ultrasonic transducer array assemblyof claim 13, wherein the porous foam material further comprises agraphite foam material.
 15. The ultrasonic transducer array assembly ofclaim 1, wherein attenuation of rearward ultrasonic emissions is due toabsorption by the epoxy resin.